In the work performed by tire dealers, it is sometimes necessary to work on tires that have different characteristics and dimensions depending on the vehicle on which they are fitted and on the use of said vehicle.
Tire inflation pressures therefore have widely different values and the machines used by tire dealers must be able to supply air at these pressures in order to be able to inflate the tires correctly, for example after replacement or other maintenance.
On the other hand, standards require tire dealers to work in conditions of absolute safety, avoiding the risks of accidental bursting normally caused by the introduction of air in overpressure in the tires.
Such pressures can be supplied by a tank, with which tire changing machines are normally provided and in which the air is kept at values on the order of 9-10 bars, or by an external supply source.
A limiting valve fitted on the pneumatic circuit of the machines downstream of said tank or of the coupling to said external supply source allows to reduce said pressure to the nominal values set by tire manufacturers, normally intended for cars or light utility vehicles, on the order of 3-3.5 bars; this reduction is explicitly necessary because the introduction of air at pressures above said values would cause deformation of the carcass of said tires.
However, in the use of said machines to inflate the tires of trucks, so-called derivative vehicles or building-yard machines, the pressures required are significantly higher, on the order of 6 bars, and therefore said machines must be equipped not only with appropriate and mandatory safety systems designed to lock the wheels thereon in case of accidental bursting of the tires during inflation, but also with devices that allow to supply compressed air at both of said values and as required.
These known devices substantially consist of a valve element that is interposed on the pneumatic circuit between the tank and the so-called inflation gun and has two operating positions that can be selected by the operator: a first position, on a lower pressure value, which is generally suitable for car tires, and a second higher value for tires of trucks and building-yard machines.
However, during normal work, between one operation and the next said valve element may be accidentally left in the position for supplying air at higher pressure, i.e., around 6 bars, while the tire dealer is about to inflate a car tire, for which the maximum allowed pressure is approximately 3.5 bars, with consequent instant damage to the carcass of the tire due to the pressure shock or, worse still, with bursting of said tire and severe risk for the safety of the tire dealer, who is practically in direct contact with the machine and with the tire.
As an alternative, if said valve elements are not used on a single machine, it is necessary to have two machines for the same function, for example for fitting and removing tires, one of said machines being provided with a pneumatic system set to a lower pressure value and one being provided with a pneumatic system set to the higher pressure value: in this case, as mentioned, it is necessary to provide specific burst protection devices in association with the machines.
In this second case, the costs for this availability are doubled.